1. Field of the Invention
The present invention relates to superresolution optical pickup apparatuses, and more particularly, to a superresolution optical pickup apparatus that records a signal at high density on a magneto-optical recording medium, and reproducing a signal from the magneto-optical recording medium.
2. Description of the Background Art
A magneto-optical recording medium is known as a recording medium that allows recording and/or reproduction of a signal at high density. In such a medium, the track includes a land portion and a groove portion. Both the land and the groove allow recording or reproduction.
Recording of a domain having a shortest domain length as small as approximately 0.15 .mu.m is now achieved with the recording capacity as large as 7 Gigabytes. In accordance with such an increase in the recording density, the spot diameter of the laser beam must be reduced in order to reproduce a recorded signal. In order to reduce the spot diameter of a laser beam directed to the signal recording face, an optical superresolution method is employed which generates a main beam and side beams by blocking the center area of the laser beam, and directing the beam onto a magneto-optical recording medium.
In Hori et al., "CAD-MSR Disk Reading with Superresolution Optical Head", Papers of Technical Meeting on Magnetics, MAG-96-200, IEEE Japan, Nov. 29, 1996, pp. 1-8, a superresolution optical head is disclosed including a liquid crystal shutter for generating a main beam and side beams by blocking the center portion of a laser beam. However, this document is absolutely silent about the specific structure of the liquid crystal shutter, the direction of polarization of the laser beam, and the like.
In general, liquid crystal rotates the plane of polarization of a laser beam passing therethrough. Therefore, the usage of liquid crystal for the shutter in an optical pickup apparatus for a magneto-optical recording medium poses the following problems. In order to reproduce a signal from a magneto-optical recording medium, the Kerr angle of rotation between the incident laser beam to the magneto-optical recording medium and the laser beam reflected from the magneto-optical recording medium is detected. It is therefore necessary to rotate the plane of polarization at an extremely high accuracy to reproduce a signal using a laser beam having the plane of polarization rotated by liquid crystal. In practice, the angle of rotation differs for every liquid crystal even if the same type of liquid crystal is used. Therefore, there was a problem that the reproduction characteristic of the magneto-optical recording medium is degraded.
Japanese Patent Laying-Open No. 5-120720 discloses an optical head having the effective numerical aperture of the objective lens altered by partially blocking the laser beam using liquid crystal and an analyzer. However, this optical head has the outer portion of the laser beam blocked to realize compatibility with optical recording media that has various recording densities.